Tetraphenylethene-functionalized diketopyrrolopyrrole solid state emissive molecules: enhanced emission in the solid state and as a fluorescent probe for cyanide detection

Abstract

Two conjugates of tetraphenylethene (TPE) and diketopyrrolopyrrole (DPP) were synthesized and their fluorescence properties were investigated. Both DPP1 and DPP2 show intense fluorescence and large Stokes shifts in both solution and solid state. In dilute THF, DPP1 and DPP2 show emission peaks at 606 nm and 632 nm with a Φ_f value of 32% and 11%, respectively. However, the powder samples for DPP1 and DPP2 emit at 650 nm and 615 nm with a Φ_f value of 13% and 7%, respectively. Moreover, DPP2 shows remarkable selectivity and specificity toward CN⁻ over other anions. The color changed from pink to colorless when CN⁻ was added to DPP2 in THF. The maximum emission intensity at 632 nm is quenched 94.6%, which constitutes the fluorescence signature for cyanide detection. The detection limit is 0.3 μM using the fluorescence spectra changes, which is far lower than the WHO guideline of 1.9 μM.

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1. Introduction

As strongly fluorescent heterocyclic pigments, diketopyrrolopyrrole (DPP) and its derivatives have been extensively applied in fluorescent probes,¹ and their structures can be easily optimized through variations of substituents at the 2,5- and 3,6-positions. They exhibit large extinction coefficients, high fluorescence quantum yields in dilute solution, brilliant red-fluorescence emission and exceptional light, weather, and heat stability. As a consequence of the well-conjugated DPP core, however, they display a typical aggregation-caused quenching (ACQ) effect. It is a great challenge to sustain the efficient emission of DPP-based compounds in aggregated states due to the serious intermolecular π–π stacking, making realization of their full potential a daunting task. To overcome this problem, it is a rational strategy to attach bulky or aggregation-induced emission (AIE) substituents onto DPP to hinder the ACQ effect.^2–4 Despite much effort, highly efficient solid emitters based on DPP are very limited.

Tetraphenylethene (TPE) and its derivatives are representatives of AIE active molecules, which possess prominent AIE properties and enjoy the easiness to be synthesized.⁵ TPE is also used to modify traditional luminous such as triphenylamine, pyrene, and perylenebisimide and successfully converted them from ACQ to AIE molecules.⁶ However, in 2014, Tang's group reported direct linking of TPE to DPP could not convert the DPP derivative from an ACQ to an AIE active molecule and obtained a DPP-based fluorescent material with high quantum efficiency in the solid state.⁷ The assimilation of DPP to TPE played a key role. Thus, it is a challenging task to examine the availability of tuning the emission performance of DPP by rational modifications.

Promoted by the previous report on conjugates of DPP and TPE by Tang's group,⁷ we were interested in the further modification of this skeleton to tune its photophysical properties and explore potential applications. It is reasonable that the insertion of a spacer between DPP and TPE moieties may break the assimilation effect. A series of excellent studies have shown that the twisting phenyl ring plays a key role in the organic photo/electronics.⁸ Moreover, the double bond is an essential feature for many AIE-active materials and its rigidity in the solid-state prevents the surrounding groups from excessive rotational or vibrational motions.⁹ For instance, in our previous work, creation of highly efficient solid emitter by decorating a diphenyl-DPP core with anthracenone by carbon–carbon double bond linker was achieved.⁴ Herein, two conjugates containing a diphenyl-DPP core and two TPE arms were synthesized, where the electronic communication between DPP and TPE is crucial for tuning their photo/electronic properties. For DPP1, vinyl group, a π-conjugate building block is used as the spacer, which can extend conjugate structure and also induce red-shifted emission. For DPP2, an extended π-deficient acrylonitrile spacer was used to blocking the good planarity to optimize its optical properties. The fluorescent behaviors of DPP1 and DPP2 in solution and aggregated sate were investigated, revealing both dyes with intense fluorescence both in solution and solid state. Due to presence of π-deficient acrylonitrile spacers in DPP2 and exceptional nucleophilicity of cyanide, DPP2 can be used as a colorimetric and fluorescent probe for detection of cyanide ions based on possible nucleophilic addition of CN⁻ to the double bond. DPP2 showed remarkable selectivity and specificity toward CN⁻ over other anions.

2. Experimental

2.1 Chemicals and instruments

Nuclear magnetic resonance spectra were recorded on Bruker Avance III 400 MHz and chemical shifts are expressed in ppm using TMS as an internal standard. The UV-vis absorption spectra were recorded using a Helios Alpha UV-vis scanning spectrophotometer. Fluorescence spectra were obtained with a Hitachi F-4500 FL spectrophotometer with quartz cuvette (path length = 1 cm). XRD patterns were collected on a D8 ADVANCE (Bruker).

The absolute fluorescence quantum yield was measured using a Hamamatsu absolute PL quantum yield spectrometer C11347, which consists of an excitation light source based on a xenon arc lamp, an integrating sphere used to measure all luminous flux, and a high-sensitivity multichannel detector. The integrating spheres are generally used to collect the emitted light. The use of integrating spheres has usually required a laser as the excitation source in combination with a fibre coupled CCD camera or a calibrated photodiode as the luminescence detectors. We can fit an integrating sphere into the sample chamber of both the FluoroLog and FluoroMax Spectrofluorometers to measure solid state photoluminescence quantum yields. This approach significantly simplifies the experimental method as the need for special equipment on the excitation and detection side is relaxed.

The THF solutions of anions (F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, HSO₄⁻, H₂PO₄⁻, NO₃⁻, ClO₄⁻) were prepared from their tetrabutylammonium salts.

Benzophenone, phenyl(p-tolyl)methanone and (2-bromoethene-1,1,2-triyl)tribenzene were purchased from Sigma. 2,5-Dioctyl-3,6-bis(4′-formylphenyl)pyrrolo[3,4-c] pyrrole-1,4-dione (compound 4) and (2Z,2′Z)-3,3′-((2,5-dioctyl-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis(4,1-phenylene))bis(2-(4-(1,2,2-triphenylvinyl)phenyl)acrylonitrile) (compound 8) were synthesized according to our published literature.⁴ (2-(p-Tolyl)ethene-1,1,2-triyl)tribenzene (compound 5) (2-(4-(bromomethyl)phenyl)ethene-1,1,2-triyl)tribenzene (compound 6) and 4,4,5,5-tetramethyl-2-(1,2,2-triphenylvinyl)-1,3,2-dioxaborolane (compound 9) was synthesized according to the literatures reported, respectively.^10–12 The recognition between DPP2 and different anions was investigated by UV-vis and fluorescence spectroscopy in THF solution at room temperature. Other solvents were obtained from commercially available resources without further purification.

The stock solution of DPP2 in THF and anions was at a concentration of 10.0 mM. After the DPP2 and anions with desired concentrations were mixed, they were measured by UV-vis and fluorescence spectroscopy.

2.2 Synthesis

2.2.1 Synthesis of compound 7. A mixture of compound 6 (650 mg, 1.53 mmol), triphenylphosphine (1.203 g, 4.6 mmol) and DMF (10 mL) were added into a 25 mL round-bottomed flask. The flask was evacuated under vacuum and plushed with dry nitrogen three times. After stirring at 95 °C for 24 h, the solution was cooled to room temperature and poured into large amount of diethyl ether. The 1.030 g white precipitate compound 7 was collected in 98% yield. Mp 262–263 °C.

¹H-NMR (CDCl₃, 400 MHz, δ), 7.79–7.60 (m, 15H, Ph-H), 7.10–6.83 (m, 19H, Ph-H), 5.31 (d, 2H, –CH₂). ¹³C-NMR (CDCl₃, 100 MHz, δ), 162.5, 144.2, 143.5, 143.0, 142.9, 141.5, 139.8, 135.0, 134.3, 134.2, 131.6, 131.2, 131.1, 130.8, 130.8, 130.1, 130.0, 127.7, 127.6, 127.6, 126.6, 126.6, 126.5, 124.8, 117.9, 117.1, 36.5. HRMS (ESI, m/z), [M − Br]⁺ calcd for (C₄₅H₃₆P), 607.7268, found, 607.7261.

2.2.2 Synthesis of compound DPP1. A mixture of compound 4 (57 mg, 0.1 mmol), compound 7 (170 mg, 0.3 mmol), K₂CO₃ (55 mg, 0.4 mmol) and 1,4-dioxane (5 mL) were added into a 25 mL round-bottomed flask. The flask was evacuated under vacuum and flushed with dry nitrogen three times. After 95 °C stirring for 3 h, the solution was cooled to room temperature. The solution was poured into water and extracted with dichloromethane. The organic layer was dried over anhydrous magnesium sulfate. After filtration and solvent evaporation, the residue was purified by column chromatography, using petroleum ether/dichloromethane/ethyl acetate (15 : 5 : 1 by volume) as eluent. Compound DPP1 (59 mg) was obtained as a bright red solid in 48% yield. Mp 238–239 °C.

¹H-NMR (CDCl₃, 400 MHz, δ), 7.84 (d, 4H, Ph-H), 7.61 (d, 4H, olefin H), 7.29 (s, 4H, Ph-H), 7.13–7.05 (m, 38H, Ph-H), 3.79 (t, 4H, N–CH₂), 1.64–1.21 (m, 30H, –CH₂, –CH₃). ¹³C-NMR (CDCl₃, 100 MHz, δ), 162.8, 147.8, 143.9, 143.6, 141.3, 140.4, 140.2, 134.8, 131.8, 131.4, 131.3, 130.6, 129.1, 127.8, 127.7, 127.6, 127.3, 127.0, 126.7, 126.6, 126.5, 126.1, 109.9, 42.1, 31.7, 29.4, 29, 29.0, 26.7, 22.6, 14.0. [M]⁺ calcd for (C₉₀H₈₄O₂), 1225.64, found, 1225.68.

2.2.3 Synthesis of compound DPP2. A mixture of compound 8 (77 mg, 0.1 mmol), compound 9 (115 mg, 0.3 mmol), and Pd(PPh₃)₄ (2 mg) were added into a 100 mL two necked round-bottomed flask. The flask was evacuated under vacuum and flushed with dry nitrogen three times. THF (10 mL) and potassium carbonate solution (2 M, 5 mL) were injected into the flask and the mixture was refluxed overnight. The solution was poured into water and extracted with dichloromethane. The organic layer was dried over anhydrous magnesium sulfate. After filtration and solvent evaporation, the residue was purified by column chromatography, using dichloromethane/petroleum ether (1.2 : 1 by volume) as eluent. Compound DPP2 (37 mg) was obtained as a bright red solid in 29% yield. Mp 227–228 °C.

¹H-NMR (CDCl₃, 400 MHz, δ), 8.03 (d, 4H, Ph-H), 7.94 (d, 4H, Ph-H), 7.50 (s, 2H, olefin H), 7.46 (d, 4H, Ph-H), 7.14–7.05 (m, 34H, Ph-H), 3.79 (t, 4H, N–CH₂), 1.62–1.21 (m, 30H, –CH₂, –CH₃). ¹³C-NMR (CDCl₃, 100 MHz, δ), 161.5, 146.5, 144.5, 142.3, 142.2, 141.2, 138.8, 138.6, 135.2, 131.1, 130.9, 130.3, 130.3, 130.2, 128.6, 128.2, 126.9, 126.8, 126.6, 125.8, 125.7, 124.4, 116.6, 112.1, 109.7, 41.2, 30.7, 28.5, 28.0, 28.0, 25.7, 21.5, 13.04. [M + Na]⁺ calcd for (C₉₂H₈₂N₄NaO₂), 1297.644, found, 1297.439.

2.3 Computations

Calculations reported in this paper were performed using Gaussian 09 program. Geometry optimizations for DPP1 and DPP2 were determined with Becke's three parameter density functional (B3LYP) method using the (6-311GIJd, p) basis set. The local minimum for the optimized structure was confirmed through absence of negative frequency.

3. Results and discussions

3.1 Design and synthesis

The molecular structures of DPP1 and DPP2 and their synthetic routes are shown in Scheme 1. Intermediate (compound 4) was prepared according to the literature.⁴ To increase the solubility, the lactam groups were alkylated upon reaction with 1-bromoctane. DPP1 was prepared by Wittig reaction between compound 4 and TPE-based yield (compound 7). To introduce π-deficient acrylonitrile spacer, compound 4 reacted with para-bromobenzylcyanide to yield compound 8. DPP2 was prepared by Suzuki coupling reaction between compound 8 and 1,2,2-triphenylethene boronic ester (compound 9). The purity of the resultants and intermediates has been confirmed by NMR spectroscopy and MS spectrometry (Fig. S1–S9, ESI†). Due to the N-modified octyl chain, DPP1 and DPP2 have good solubility in common organic solvents. This allows their electronic structures to be evaluated with optical absorption, fluorescent emission measurements in solution such as THF, chloroform (CHCl₃), however, they display poor solubility in hexane and polar solvents such as methanol and water.

Scheme 1 Synthesis of DPP1 and DPP2.

3.2 Photophysical properties of DPP1 and DPP2 in solution and solid state

In THF, the maximum absorption band of DPP2 at 526 nm shows a red shift of 13 nm compared to that of DPP1 (513 nm) due to the electron-withdrawing cyano groups of DPP2, which is typical for the S₀–S₁ transition of DPP chromophore (Fig. S10a, ESI†). The second absorption band (300–400 nm) for DPP1 and DPP2 with peak maximum at 355 and 340 nm, respectively, is due to a π–π* transition involving both the DPP and TPE units. In the fluorescent spectra, the maximum emission wavelength of DPP2 is observed at 632 nm, which is 26 nm longer than that of DPP1 (Fig. S10b, ESI†). It is interestingly found that both DPP1 and DPP2 exhibit a large Stokes shift even up to 100 nm. The results demonstrate that the electron-donating TPE substituent introduced to the DPP core by different spacer affects the electronic structure of DPP1 and DPP2.

The emission study of DPP1 in different solvents indicates that the luminescent properties are slightly affected by solvent polarity (Fig. S11, ESI†). The emission band has no obvious red-shift, but its intensity is reduced with an increase in the solvent polarity. The FL spectra of DPP2 show similar features as compared to that of DPP1 in different solvents (Fig. S12, ESI†).

The property that we are most interested in is their fluorescent behaviors in aggregated states. So, we investigated the fluorescent behaviors of DPP1 and DPP2 in CHCl₃/hexane, CHCl₃/methanol and THF/water with different volume percentage of hexane (f_h), methanol (f_m), and water (f_w), respectively. Typically, Fig. 1a shows the FL spectra of DPP1 in CHCl₃/hexane. In CHCl₃, DPP1 emits orange fluorescence (λ_em = 605 nm) and the quantum efficiency (Φ_f) is 33%. With increase of f_h, the strong emission of orange characteristic band is gradually enhanced. Φ_f is 44% at f_h of 90% (Fig. 1b). Besides the increase in PL intensity, the emission peak showed slightly blue-shift from 605 nm to 602 nm with the increase of f_h from 0 to 90%. The emission photographs of DPP1 in CHCl₃/hexane mixtures with different f_h values are shown in Fig. 1c. Different fluorescence effect of DPP1 in CHCl₃/methanol mixtures is present (Fig. 2). When f_m < 40%, the emission intensity monotonously increases to negligible. When f_m > 40%, aggregates form in the system and the emission intensity greatly decreases. The Φ_f is only 9% at f_m of 90%.

Fig. 1 (a) The photoluminescence (PL) spectra, (b) PL intensity vs. f_h and (c) emission photographs of DPP1 in CHCl₃/hexane mixtures with different f_h values.

Fig. 2 (a) The photoluminescence (PL) spectra, (b) PL intensity vs. f_m and (c) emission photographs of DPP1 in CHCl₃/methanol mixtures with different f_m values.

As demonstrated by the FL spectra in Fig. 3, in THF solution, DPP1 emits orange fluorescence (λ_em = 605 nm) with Φ_f of 32%. When relative low fraction of water (f_w, by volume, 40%), a non-solvent to DPP1, is introduced into the THF solution, slightly enhancement in emission intensity is present. When f_w reaches 50%, the emission intensity decreases greatly. The change in fluorescence intensity vs. f_w is shown in Fig. 3b. The Φ_f is only 3% at f_w of 90%. The decrease of quantum efficiency against the increase of f_w can be ascribed to the formation of aggregates in the THF–water mixtures, which quenches the fluorescence via the intermolecular π–π interaction. Besides the decrease in emission intensity, the emission peak red-shifted from 605 to 619 nm with the increase of f_w from 0 to 90%. Such an observation clearly indicates that the hydrophobic DPP1 molecules had formed large aggregates, as both water/THF are hydrophilic solvents and only those smaller ones in the suspension contributed to the emission. To understand mode of aggregation, we have performed UV-vis absorption spectra of DPP1 in THF/water (Fig. S13, ESI†). Aggregation effect in which a reduction in the peak intensity and significant red-shift of the absorption maximum along with loss of fine structure are seen in THF/water as compared to bands in THF alone. The absorption tails extending well into the long wavelength region indicate that DPP1 aggregate into particles in the presence of water, as it is well known that the Mie effect of nanoparticles can cause such level-off tails in the absorption spectra.¹³

Fig. 3 (a) The photoluminescence (PL) spectra, (b) PL intensity vs. f_w and (c) emission photographs of DPP1 in THF/water mixtures with different f_w values.

The PL spectra of DPP2 clearly show similar features as compared to that of DPP1 in different solvent mixtures. As shown in Fig. S14 (ESI†), the emission of DPP2 in the CHCl₃ solution is strong with an emission maximum at 628 nm (Φ_f = 15%). When hexane, a non-polar and poor solvent to DPP2, is introduced into the CHCl₃ solution, the PL signal is gradually enhanced upon increasing hexane/CHCl₃ ratios. When f_h reaches 80%, the PL intensity is boosted to the maximum. The quantum efficiency of DPP2 is 1.3-fold higher than its original emission intensity in CHCl₃. However, the emission intensity monotonously decreases when polar and poor solvent of methanol is added to CHCl₃ solution (Fig. S15, ESI†). The Φ_f is only 4% at f_m of 90%. It is noted that the emission peak gradually blue-shifts from 628 to 624 nm when f_m is increased from 0 to 90%. Such a spectral shift can be explained as follows: with increased methanol content, DPP2 molecules start to aggregate, which generates less polar microenvironment for the molecules inside the aggregates. The reduced environment polarity leads to the observed blue shift in the emission spectrum.¹⁴ The similar behavior was also observed for THF/water mixtures, only 4% Φ_f was shown in presence of 90% water (Fig. S16, ESI†).

Usually, emission of DPP derivatives is never observed in the solid state, because the emission is completely quenched by π–π interaction in aggregation. As expected, no emission was observed for pure compound 4 as shown in Fig. S17 (ESI†). It is intriguing to note that DPP1 and DPP2 still retain intense fluorescence even in the powder form. This observation prompted us to investigate their photophysical properties in the solid state in details. The emission spectra, together with photographs taken under UV illumination, are shown in Fig. 4. Upon exciting at 365 nm, DPP1 in powder emits strongly in the red region. With respect to corresponding emission at 606 nm in THF solution, the fluorescence maxima peak at 650 nm for DPP1 in powder is obviously red-shifted. However, it is quite surprising that the emission maximum of DPP2 in powder is located at 615 nm, which is hypochromic shift of 17 nm than that in THF. This indicates that the DPP2 fluorophores are still in their isolated form in the powder without heavy intermolecular interactions, as strong π–π overlaps in the aggregated or solid state always causes delocalization of the excited states and induces emission at a longer wavelength.¹⁵ However, this is not true for DPP1. Evidently, the highly distorted and rigidified conformation of the TPE unit in DPP1 and DPP2 may lead to an extended π–π interplanar distance and/or inefficient π–π overlap when molecules are aggregated.

Fig. 4 Normalized photoluminescence (PL) spectra of DPP1 and DPP2 in powder. Inset: color and emission photographs of DPP1 and DPP2.

The fluorescence quantum efficiency (Φ_f) of the samples was estimated using Rhodamine 6G (Φ_f = 95% in ethanol) as standard solution and Φ_f of the solid film was measured using an integrating-sphere photometer. DPP1 (10⁻⁵ M) in cast film shows a fluorescence quantum yield (Φ_f) 13%, which is lower than its Φ_f in CHCl₃ and THF i.e. 33%, 32% respectively. Interestingly, DPP2 shows quite different behavior than DPP1. It is found that the Φ_f of DPP2 in cast film is 7%, which is lower than Φ_f in CHCl₃ and THF is 15%, 11%, respectively. However, Φ_f of DPP1 and DPP2 in solid state are higher than that of DPP–triphenylamine dyes (Φ_f = 0.03%, 0.1%),^3b DPP–TPE dye with donor–acceptor effect (Φ_f = 8.6%)⁷ and others.¹⁶ It should be noted that the introduction of phenylacrylonitrile leads to a larger Stokes shift but a lower Φ_f both in solution and sold-state as compared with DPP1. The corresponding photophysical data are shown in Table 1.

Table 1 Photophysical data of DPP1 and DPP2 in solution and solid state

	HOMO (ev)	LUMO (ev)	Gap (ev)	THF^a			Solid-state
				λab (nm)	λem (nm)	Φf^b	λem (nm)	Φf^c
a Measured at a concentration of 10 μM at 25 °C.b Estimated using Rhodamine 6G (Φf = 95% in ethanol) as reference.c Absolute quantum yield determined by calibrated integrating sphere systems.
DPP1	−5.17	−1.69	3.48	513	605	32%	650	13%
DPP2	−5.38	−2.21	3.17	526	632	11%	615	7%

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The morphology of DPP1 and DPP2 powders were further investigated by X-ray diffraction (XRD), which show many sharp and strong peaks, indicative of their crystalline nature (Fig. 5). In our case, the molecules of DPP1 and DPP2 may steadily cluster together to form ordered microstructures and hard to be changed due to the restriction of the crystal lattice, and this may make the crystals stronger emitters because of the weakened π–π interactions among the fluorophores. These results will provide the way to tune the performance of organic luminogens by making subtle balance between AIE and ACQ units; and on the other hand, derived red-emitting molecules with good quantum efficiency.

Fig. 5 XRD patterns of DPP1 and DPP2 in solid powder.

3.3 Theoretical calculations

The red-shift of emission of DPP1 and DPP2 relative to compound 4 (DPP–aldehyde in THF, λ_ab = 497 nm, λ_em = 592 nm) is associated with the segmental electronic conjugation between DPP and TPE moieties, as predicted by theoretical calculation (Fig. S18, ESI†). In the free state of DPP1, the dihedral angle between the vinyl bridge and the adjacent phenyl in the TPE moiety is about 2.966°, suggesting the good conjugation between the TPE and DPP moieties. In comparison with DPP1, the large dihedral angle between the acrylonitrile bridge and the adjacent phenyl in the TPE moiety is 26.739° for DPP2. Moreover, the dihedral angles between the vinyl bridge and adjacent another phenyl adjacent to DPP core are 4.174° and 9.637° for DPP1 and DPP2, respectively. These results indicate that the addition of the acrylonitrile groups between DPP and two TPE moieties reduces planarity of DPP2.

To further elucidate the effect of the spacer on the electronic structures and thus the photophysical properties of the DPP dyes, we conducted total energy calculation using density functional theory (DFT) calculations at the B3LYP/631G(d) level of theory. The density maps of their molecular orbitals are shown in Fig. 6, and the calculated data are summarized in Table 1. DPP1 has a HOMO delocalized over the vinylbenzene moiety and the entire TPE arms, whereas its LUMO is mostly localized on the central DPP and vinylbenzene moieties. The HOMO and LUMO energy levels are −5.17 eV and −1.69 eV, respectively. It is noteworthy that in sharp contrast to the remarkable contribution of vinylbenzene moiety to the LUMO for DPP1, DPP2 has a HOMO delocalized over the entire TPE arms, whereas its LUMO is mostly localized on the central DPP, benzene and acrylonitrile moieties. The HOMO and LUMO energy levels are −5.38 eV and −2.21 eV, respectively. The decreased HOMO–LUMO gap of DPP2 should be responsible for the red-shifted main emissive band relative to that of DPP1. All the calculated results are in good agreement with the experimental results.

Fig. 6 Energy level alignment and electron distribution of DPP1 and DPP2.

3.4 Detection of cyanide with DPP2

Cyanide has been continuously of concern all over the world due to its high toxicity and widespread use in industrial processes such as acrylic fiber manufacturing, metallurgy, herbicide production, and silver or gold extraction.¹⁷ Therefore, it is of great significance to find convenient and efficient detection methods at the environmental and biological levels. The burgeoning field of reaction-based indicators has made progress in this area because of the unique reactivity of CN⁻ toward a variety of organic functional groups including C=O, C=N, C=C, and so on.¹⁸ The irreversible formation of chemical bonds can provide chemodosimetric information and develop ratiometric fluorescent probes. It is well known that general Michael acceptors, the doubly activated acceptors, in which two electron-withdrawing groups are attached to the C=C group, are more reactive, and allow the Michael addition reaction to occur under mild condition. Clearly, to improve the sensitivity, it is necessary to enhance the reactivity of the probe to cyanide. With this in mind, we recently reported chemodosimeter approaches based on diketopyrrolopyrrole, where electron withdrawing cyano and ester groups or 1,3-indanedione to afford doubly activated Michael addition type probes for the colorimetric fluorescent detection of cyanide with high sensitivity and good selectivity.¹⁹

The interaction of DPP1 in THF with cyanide through absorption and fluorescence spectra titration experiments was carried out. As shown in Fig. S19–S20 (ESI†), no changes could be found in presence of cyanide. DPP1 is inactive to cyanide due to low reactivity of vinyl group in DPP1. However, DPP2 may have great potential for detection of cyanide due to presence of π-deficient phenylacrylonitrile section, electron withdrawing cyano group connected with the α-position of the ethylenic bond. The interaction of DPP2 in THF with cyanide through absorption and fluorescence spectra titration experiments was carried out. DPP2 exhibits main absorption bands centered at 336 and 525 nm, respectively (Fig. 7). Upon addition of 0–24.0 equiv. of cyanide, both bands gradually decrease and the one at 525 nm eventually vanishes at the saturation point (24.0 equiv.). Two new bands at 312 nm and 360 nm are present. This change suggests that the large π-conjugation is interrupted, accompanied by the formation of [DPP2–CN]²⁻ due to the nucleophilic attack of cyanide toward the acrylonitrile groups of DPP2. Moreover, the solution faded and became almost colorless, which could be clearly observed by naked eye (Fig. 7 inset).

Fig. 7 UV-vis spectral changes of DPP2 in THF (10 μM) with the increasing concentrations of cyanide anion.

As expected, the fluorescence emission of DPP2 is quenched by the nucleophilic addition reaction between cyanide and acrylonitrile group. Upon excitation at 520 nm, a broadened emission band at around 632 nm is detected, which gradually decreases upon addition of cyanide and became saturated with 52.0 equiv. of CN⁻ (Fig. 8). Finally, 94.6% PL intensity at 632 nm is quenched. Additionally, a linear relationship between the fluorescent intensity at 632 nm and the concentration of CN⁻ in the low concentration region is obtained (Fig. S21, ESI†). The detection limit is as low as 0.3 μM, which is much lower than the maximum contaminant level for CN⁻ in drinking water (1.9 μM) set by the World Health Organization (WHO).²⁰ In comparison with reported probes,²¹ the detection limit of DPP2 is moderate for fluorescence detection limits to cyanide.

Fig. 8 The photoluminescence (PL) spectral changes of DPP2 (10 μM) in THF with the increasing concentrations of CN⁻ in THF under excitation at 520 nm.

For the purpose of evaluating selectivity of DPP2 to cyanide, the absorption spectral change of DPP2 upon addition of other anions was also investigated. Dramatic change of the absorption spectrum induced by CN⁻ was observed, while almost no changes could be found in presence of other anions, including F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, HSO₄⁻, H₂PO₄⁻, NO₃⁻, ClO₄⁻ (Fig. 9a). More importantly, the color change from pink to colorless can be clearly observed by the naked eye in presence of CN⁻, while other anions did not induce any significant color change, which suggested that naked-eye selective detection of CN⁻ became possible (Fig. 9b).

Fig. 9 (a) UV-vis absorption spectra and (b) color changes of DPP2 in THF (10 μM) upon addition of 24 equiv. of each anion.

Meanwhile, only CN⁻ renders a remarkable “Turn-Off” fluorescence response, whereas all other anions reveal a negligible change in the fluorescent spectra of DPP2 (Fig. 10a). In the presence of 52 equivalents of CN⁻, an emission peak at 632 nm with 94.6% fluorescence quenching is observed. Fig. 10b showed the photographs of DPP2 in the presence of different anions under excitation at 365 nm using a portable UV lamp. The disappearance of intense red color of the solution upon interaction of DPP2 with CN⁻ is present. However, other anions do not induce any significant emission color change.

Fig. 10 (a) Photoluminescence (PL) and (b) emission color changes of DPP2 in THF (10 μM) upon addition of 52 equiv. of each anion aqueous solution.

Another important feature of DPP2 is its high selectivity toward the CN⁻ in presence of other competitive anions. Changes of fluorescence response of DPP2 (10 μM) caused by CN⁻ (52 equiv.) and miscellaneous competing species (52 equiv.) were recorded in Fig. S22 (ESI†). As can be seen, these competitive species, did not lead to any significant interference. In the presence of these ions, the CN⁻ still produced similar optical spectral changes. These results showed that the selectivity of DPP2 toward CN⁻ was not affected by the presence of other anions.

Fig. S23 (ESI†) showed the time-dependent absorbance at 525 nm of DPP2 in the presence of CN⁻ (24 equiv.) at room temperature in THF. Generally, reaction-based probes suffer from a long response time. In our case, the response of DPP2 to CN⁻ is found to be fast. The absorption peak of DPP2 at 525 nm gradually decreased when 24 equiv. of CN⁻ is added with time passing. After 15 minutes the UV-vis spectra of DPP2 are unchangeable, indicating nucleophilic addition reaction between the vinyl group and CN⁻ is completed, which is impressive as many reported cyanide probes require high equivalents of cyanide and long reaction time to reach a maximal spectral signal.^22–31

3.5 Practical application

Active materials-based test strips represent a group of convenient probing substrate for practical utilization. DPP2-based test strip was thus fabricated by immersing filter paper into the THF solution of DPP2 (1.0 × 10⁻³ M) and drying in air, which was energy- and cost-effective. The corresponding probing experiments were carried out subsequently. The results indicated that this protocol really took effect. The obvious color change from pink to colorless was observed by immersing this test strips in solutions of CN⁻, exhibiting colorimetric changes differentiable to naked eyes (Fig. 11a). As shown in Fig. 11b, when the DPP2-exposed test strip was immersed into solutions of CN⁻ (0.001 M), strong red fluorescence disappeared and easily distinguished.

Fig. 11 (a) Color and (b) emission changes of DPP2-based test strips before and after addition of CN⁻.

4. Conclusions

In summary, we described synthesis of two new TPE substituted DPP derivatives (DPP1 and DPP2) and studied comparative fluorescence behaviors. Typically, DPP1 in dilute THF solution shows the characteristic PL features (λ_em = 606 nm) with Φ_f 32%. Its powder sample emits red fluorescence (λ_em = 650 nm, Φ_f = 13%). DPP2 in dilute THF solution has Φ_f = 11% (λ_em = 632 nm) and its powder sample emits red PL (λ_em = 615 nm, Φ_f = 7%). Moreover, DPP2 displays high selectivity and sensitivity toward CN⁻. The strong red emission of DPP2 was completely quenched in the presence of CN⁻. However, other anions did not induce any significant change. Test strips for use in practical applications are successfully realized.

Acknowledgements

The supports by National Basic Research Program of China (2012CB720801), National Natural Science Foundation of China (No. 21274045), the Fundamental Research Funds for the Central Universities (2015ZZ037) and the Natural Science Foundation of Guangdong Province (2015A030313209, 2016A030311034) are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10073b

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